1. Field of the Invention
The invention relates generally to radio receivers, and more specifically to radio receivers that are used for wireless telephone communications.
2. Description of Related Art
The field of wireless communications is experiencing rapid growth. In order to increase the capacity of existing cellular communications systems, present efforts are being directed towards new modulation schemes, such as CDMA and TDMA. One characteristic of CDMA is that relatively broadband radio receiver designs must be employed. At the same time that these modulation schemes are enjoying more widespread acceptance, new cell sites and other radio communications systems are being constructed at an ever-increasing pace. Consequently, wireless telephone receivers must be able to effectively discriminate against many sources of interference.
The extent to which a wireless telephone receiver is immune to RF interference from nearby transmitters is determined by the intermodulation distortion characteristics of the receiver. As a general matter, broadband receivers are more susceptible to intermodulation distortion than narrowband receivers. These intermodulation distortions occur at the receiver front-end when the front end is exposed to strong undesired out-of-band signals as, for example, when the receiver is in close proximity to a cell site transmitter other than that from which the receiver is currently receiving a radio signal.
One conventional technique for improving intermodulation distortion problems addresses the design of the receiver front end. As a general matter, intermodulation performance can be improved by increasing the quiescent operating current of the active RF device or devices in the receiver front end. In order to obtain acceptable intermodulation performance in many real-world environments, the quiescent operating current must often be increased to an undesirably high level. In the case of stationary equipment connected to a 120-volt mains supply, the use of such a high current generally poses no great problem. However, a different situation exists with respect to portable equipment, where such a high current drain would very quickly deplete a set of fresh batteries. The user is inconvenienced by having to frequently change and/or recharge batteries. Such batteries may be expensive, and, if not, the frequent purchase of inexpensive batteries may also prove costly. Moreover, the user may be faced with a set of dead batteries in an emergency situation. What is needed is a technique for improving intermodulation performance while, at the same time, not significantly increasing the current consumption of the receiver.